Devices for pumping molten metal (referred to herein as molten metal pumps or pumping devices), particularly molten aluminum, and various components that can be used with these devices are generally disclosed in U.S. Pat. No. 2,948,524 to Sweeney et al. and U.S. Pat. No. 5,203,681 to Cooper entitled "Submersible Molten Metal Pump," the disclosures of which are incorporated herein by reference.
A problem inherent in prior art devices is costly, time-consuming maintenance. Molten metal pumping devices operate in an extremely hostile environment, usually a molten aluminum bath. The molten aluminum is maintained at a temperature of 1200-1500.degree. F. and contains contaminants, such as magnesium, iron, dross and pieces of brick. Additionally, chlorine gas, which is highly corrosive, is usually released in the molten aluminum to react with and remove the magnesium. As a result of the high temperatures and chemical composition of the metallic bath, the bath is extremely caustic and gradually oxidizes the pumping device's components.
Another problem with molten metal pumps is related to the pressure generated by pumping the metal and the presence of solid particles within the molten metal bath. Molten metal pumps include a motor, a rotor shaft, a rotor (or impeller) and a pump base. The pump base has a chamber formed therein, an input port(s) (also called an inlet(s)) and a discharge that leads to an output port (also called an outlet). The input port and discharge are in communication with the chamber. The motor is connected to the rotor shaft and drives, or spins, the rotor shaft, connected to the rotor, which is located within the pump chamber. The molten metal enters the chamber through the input port(s) and the spinning rotor forces (i.e., pumps) the molten metal through the discharge and out of the port.
The pressure generated by pumping the molten metal can cause the rotor shaft to move eccentrically (i.e. to wobble). Further, if solid particles such as slag or brick enter the pump chamber and strike the rotor, the rotor shaft is jarred. Eccentric movements and sudden changes in speed caused by jarring can damage the rotor shaft or the coupling that joins the rotor shaft to the motor drive shaft. In order to prevent the rotor shaft from breaking, and to prevent damage to the coupling, the coupling should be flexible to allow for movement.
Further, when dross, pieces of brick or other solid particles enter the pump chamber they may wedge between the rotor and the upper wall of the pump chamber, which may cause the rotor to jam and the rotor shaft to break. One solution to this problem is described in U.S. Pat. No. 5,203,681 to Cooper entitled "Submersible Molten Metal Pump." This patent discloses a pump having a non-volute pump chamber to allow for the passage of solids. Even if this design is utilized, however, solid particles may still wedge between the upper wall of the pump chamber, or upper wear ring, and the rotor, thus jamming the rotor.
Further, molten metal pumps come in several versions, one of which is referred to as a transfer pump. A transfer pump normally has a discharge formed in the top of the pump housing. A metal-transfer conduit, or riser, extends from the discharge and out of the metallic bath where it is generally supported by a metal support structure known as a superstructure and is connected to a 90.degree. elbow. The transfer pump pumps molten metal through the discharge and through the metal-transfer conduit and elbow where it exits into another metallic bath chamber (i.e., the molten metal is transferred to another chamber). Until now, the metal-transfer conduit has been cemented to the discharge opening and to the steel superstructure. Although cementing the conduit generally works well, it is extremely difficult to replace a metal-transfer conduit so connected because: 1) the pump must be removed from the metallic bath and cooled, 2) the cement must be chiseled away, 3) the new conduit must be assembled and cemented to the discharge, 4) the conduit must be cemented to the steel supporting structure, and 5) the new cement must be cured to remove moisture, a process that, by itself, normally takes approximately twenty four hours. The entire replacement operation can take up to two days.